Railroad grade crossing protection system

ABSTRACT

A railroad crossing warning indicator which predicts the time of arrival of trains to a grade crossing is described, which includes a quadrature detector which provides a distance voltage which is derived from the reactance magnitude, and an amplitude detector which provides a distance voltage derived from the impedance magnitude. The voltage derived from the impedance magnitude is applied to a differentiating circuit which provides the speed of the train. The reactance voltage from the quadrature detector and the impedance voltage derivative circuit from the differentiating circuit are then summed whereby the sum of the two voltages provides an improved estimate of the time of arrival of the train with the decrease in error.

United States Patent [72] Inven r lli ll Pal Primary Examiner-Arthur L.La Point La Verne, Calif. Assistant Examiner-George H. Libman [2| 1Appl. No. 807,626 Attorney-Robert E. Geauque [22] Filed Mar. 17,1969[45] Patented Sept. 7, 1971 [73] Assignee Marquardt Industrial ProductsCo.

Cueamonga, Cflii.

ABSTRACT: A railroad crossing warning indicator which pre- [54] RAILROADGRADE CROSSING PROTECTION dicts the time of arrival of trains to a gradecrossing is SYSTEM described, which includes a quadrature detector whichpro- 13 cums, 2 Drawing Figs vides a distance voltage which is derivedfrom the reactance magnitude, and an amplitude detector which provides a[52] US. Cl 246/128 distance voltage derived f the impedance magnimdeThe [51 1 In.

voltage derived from the impedance magnitude is to a 29/32differentiating circuit which provides the speed of the train. [50]Field oiSearch 246/128 Th reactance lt f th quadrature d t t d thimpedance voltage derivative circuit from the differentiating [56]Refemuces Cited circuit are then summed whereby the sum of the twovoltages UNITED STATES PATENTS provides an improved estimate of the timeof arrival of the 3,246,143 4/1966 Steele et a]. 246/128 train with thedecrease in error.

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l M I E. 52 44 IVA/MW? Ann/0412 ///4// may 0/ IMIAM/ll 44 4a m. 714a.may! IMAt/F/[Z RAILROAD GRADE CROSSING PROTECTION SYSTEM BACKGROUND OFTHE INVENTION 1. Field of the Invention This invention relates towarning systems for railroad grade crossings and more particularly to animprovement in warning predictor systems used to predict the time ofarrival of an approaching train.

2. Discussion of the Prior Art A typical grade crossing predictor is setforth in U.S. Pat.

No. 3,246,143. Much of the same system's electronics of that patent areused in the invention discussed herein.

The system of the patent provides a railroad crossing warn- 1 ing systemwhereby delay to cross traffic is minimized. This is achieved in anarrangement wherein the railroad track is considered as a shortedtransmission line in which the short is provided by the train. Analternating current signal which is a substantially constant currentlevel is applied to the tracks at the location of the grade crossing.The voltage existing across the tracks as the train, and therefore theshort, approaches the grade crossing, will diminish. Thus, the amplitudeof this voltage provides a measure of the distance of the train from thei crossing while the rate at which this voltage diminishes provides ameasure of the velocity of the train. With these parameters it becomespossible to estimate the time of the trains arrival at the crossing.Knowing the time of arrival, the

a system can start warning signals at such a time as will provide theleast possible delay to cross traffic. The signal representative ofdistance and the derived signals therefrom representative of velocityare combined to provide a third voltage 1 representative of the timerequired for the train to arrive at the 1 railroad grade crossing.

It has been found that the input impedance of the shorted i railroadtrack section, having infinitely high ballast resistance, varieslinearly with track length. The grade crossing predictor,

as set forth in the above patent, for example, uses this principle todevelop a voltage which is the measure of the distance of the train tothe predictor probe location. The voltage is a derived from thereactance component of the input impedance. The rate at which thisvoltage diminishes as a train approaches, provides a measure of thespeed of the train. These two voltages are then combined to estimate thetime of the trains arrival at the crossing. Knowing the time of arrivalof the train, the aforesaid system can initiate warning signals beforethe arrival thereof.

Since, in actual practice, ballast resistance is low enough to cause theinput impedance, and in particular the reactance component, to varynonlinearly with track length, an error is introduced into the distancevoltage and thus the speed voltage. These two errors cause the predictorto err in the estimate DESCRIPTION OF THE DRAWINGS These and otherobjects, features and advantages will become more apparent to thoseskilled in the art when taken into consideration with the followingdetailed description,

wherein like reference numerals indicate like and correspond ing partsthroughout the several views, and wherein:

FIG. 1 is a block diagram of the preferred embodiment of the invention;and

FIG. 2 is a graph of the voltage versus distance and the error reductionlinearity realized by this invention.

DESCRIPTION OF ONE PREFERRED EMBODIMENT Turning now to FIG. 1, there isshown a block diagram of the preferred embodiment of this invention. Thetrain 10 has a motion in a direction represented on a pair of trackrails l2. The train is at a distance L from the origin point P, P, whichrepresents the location of a grade crossing, for example. The trainmotion occurs from left to right. The velocity V and the acceleration Afactors are, therefore, represented as going from left to right on thedrawing. The method of computing the time of arrival is set forth fullyin the aforesaid U.S. Pat. No. 3,246,143.

A computer, in accordance with the aforesaid, includes an oscillator 16which oscillates at a suitable frequency. The output of the oscillator16 is applied to excite a power amplifier 22. A resistor 18 connects oneside of the power amplifier to one of the rails at a point P. The otherside of the power amplifier 22 connects to the other rail P. The poweramplifier 22, together with the resistor .18, comprises a constantcurrent generator. This delivers an input to the track at substantiallya constant current.

It should be appreciated that as the train 10 approaches the points P, Pon the track to which current from the constant current generator isapplied, the impedance of the tracks looking toward the train from thesepoints is continuously being diminished. Thus, the train comprises ashort across tracks 12, which is moved toward the points P, P. Withcurrent being maintained constant, the voltage at the points P, P willcontinuously decrease to a minimum when the train reaches the points P,P. Therefore, by measuring the voltage acrossthe tracks 12, anindication is obtained of the distance of the train 10 from the pointsat which the voltage is impressed. The change, with respect to time ofthis voltage, can provide velocity information and a second derivativeof this voltage provides information as to the acceleration of the train10.

Accordingly, a narrow band-pass amplifier 26 centered at the frequencyof the oscillator 16, which is connected to the same points of thetracks 12 as the constant current generator, receives a voltagerepresentative of length of track L or distance between the train 10 andthe points P, P. This voltage is an alternating current which ismodulated by the motion of the train 10 toward the points P, P.

The output of the band-pass amplifier 26 is applied to a quadraturedetector 28 which also has a reference input applied from the oscillator16 through a phase shift network 30. The output of the quadraturedetector 28 is applied to a summing amplifier 38. The output of theband-pass amplifier 26 is also coupled to an amplitude detector 32which, in turn, has an output coupled to an inverting differentiatingcircuit 34. The amplitude detector 32 provides a DC voltage proportional to the impedance of the track 12. Circuit 34 provides the rate ofchange of that impedance. The output of the circuit 34 is coupledthrough an amplifier 36 to a summing amplifier 38. The summing amplifier38 receives the time rate of change of the impedance derived distancevoltage from the output of the differentiating circuit 34 which is equalto the speed of the train 10. The output of the summing amplifier 38 isconnected to a high gain amplifier 40. The output of the high gainamplifier is applied to an amplitude comparator 42 wherein it iscompared with a signal from the reference voltage source 44. The outputof the amplitude comparator 42 is connected to a relay amplifier 46which operates the warning relay when the signal applied into it has asufficient magnitude.

An override circuit is also provided, and this includes an amplitudediscriminator 52 which receives the output from the quadrature detector28 and compares it to the output of a reference voltage source 54. Theoutput of the amplitude discriminator 52 is connected to a relayamplifier 56 which drives a minimum distance override relay 58. Theinput to the differentiator circuit 34 and summing amplifier 38 arevoltages proportional to the distance L between the train and theexcitation points P and P'. When differentiated, this voltage gives avoltage proportional to train speed. The output of the quadraturedetector 28 is a voltage proportional to the reactance component acrossthe track which is a measure of the distance to a train from points Pand P.

FIG. 2 illustrates the difference between the distance voltage derivedfrom the reactance magnitude provided by the quadrature detector 28 andthe distance voltage derived from the impedance magnitude provided byamplitude detector 32. The sum of distance voltage derived from thereactance magnitude and the time rate of change of the distance voltagederived from the impedance magnitude is provided by the summingamplifier 38.

When the ballast resistance is very high (R,,= the two distance voltageshave the same slope. When the ballast resistance is decreased (R,,=l.5ohms, for example, lumped at the predictor) it can be readily seen thatthe slope of the impedance magnitude is much improved over the slope ofthe reactance magnitude, as shown in the two graphs in FIG. 2. Thus, theerror in estimate of the arrival of a train by the predictor is alsomuch improved. As stated in U.S. Pat. No. 3,246,143, the reactanceportion of the input impedance is unaffected by a bad bond which resultsin a lumped resistance in the track circuit. Therefore, the reactancecomponent voltage is more satisfactory for obtaining a value which is afunction of train distance. However, as illustrated in FIG. 2, thereactance voltage vs. train distance for a low ballast track sectionapproaches substantially zero slope at a first train distance which isless than the desired prediction distance. On the other hand, theimpedance voltage vs. distance curve maintains a slope substantiallygreater than zero over a greater train distance. Therefore, theimpedance voltage can be differentiated over train distance greater thanthe first distance. Thus, the utilization of the reactive component formeasuring distance and the impedance component for measuring velocityprovide a more accurate prediction of time of arrival over a greaterlength of track. The reason that the impedance magnitude provided byamplitude detector 32 is less affected by low ballast resistance thanthe reactance magnitude is apparent in the following example:

Z,,,=R+J X If we assume that R=0.5X, which provides a high ballastresistance condition, then Z,,,=l.l2X 63.4 lf we assume a low ballastresistance condition (R =2X) where the ballast resistance is in parallelwith Z then Z,,,=0,83X 4l .6 therefore, the impedance magnitudeZ,,,=(0.83/ 1.12) or a 26 percent reduction in its magnitude while thereactance magnitude or a 45 percent reduction in its magnitude for thesame given' low ballast condition.

Thus, there has been provided by the improvements set forth herein atime of arrival predictor computer which has a lower error as comparedto the prior art system. The output, as provided by this predictor fromsumming amplifier 38, is sent through a high gain amplifier 40 andcompared in a comparator 42 to a reference voltage provided by 44. Ifthe sum of these voltages is above the reference voltage, then the relayamplifier 46 enables a relay 48, which, in turn, either sounds an alarmor lowers a crossing gate, or the like.

Having thus described but one preferred embodiment of this invention,what is claimed is:

1. In a system for deriving from railroad tracks of low ballastinformation for predicting the time required for arrival at a givenlocation of a distant train which is moving on said track towards saidlocation, comprising:

means for applying an AC signal of a constant current level on saidtrack from said location;

means for deriving a first voltage proportional to the reactancecomponent across said tracks said first voltage having an output vs.train distance curve approaching substantially zero slope in thevicinity of a first train distance from said location means for derivinga second voltage proportional to the impedance component across saidtracks having a time rate of change indicative of the speed of saidtrain up proaching said location, said second voltage having an outputvs. train distance curve of a slope substantially greater than zero overa train distance greater than said first train distance;

said first voltage being indicative of the distance of said train fromsaid location over said greater train distance and being substantiallyfree from the effect of lumped resistance in said track;

means for differentiating said second voltage to obtain a voltagerepresentative of the instantaneous speed of said train at any trainposition over said greater train distance; and

means for combining said first voltage and said differentiated voltageto provide a third voltage which is a function of the time required forsaid train to arrive at said given location from any train position oversaid greater train distance.

2. The system as defined in claim 1 and further comprising means forutilizing said third voltage for operating a warning device at saidlocation.

3. The system as defined in claim 1 wherein said means for deriving saidfirst voltage is a quadrature detector.

4. The system as defined in claim 1 wherein said means for deriving saidsecond voltage is an amplitude detector.

5. The system as defined in claim 1 wherein said means fordifferentiating is an operational amplifier differentiator.

6. The system as defined in claim 1 and further comprising:

means for providing a reference voltage; and

comparator means responsive to said reference voltage and said thirdvoltage provided by said combining means, said comparator means beingadapted to provide an output when said third voltage exceeds saidreference voltage.

7. The system as defined in claim 6 and further comprising means forutilizing the output voltage of said comparator means for operating awarning device at said location.

8. In a system for predicting the time of arrival of a train on a trackcomprising:

a source of AC signals at a constant current level, said source beingcoupled across said tracks at a selected location;

a quadrature detector being coupled to receive signals from said trackat said selected location, said quadrature detector being adapted toprovide a first output voltage proportional to the reactance componentacross said track and having an output vs. train distance curveapproaching substantially zero slope in the vicinity of a first traindistance from said location;

an amplitude detector being coupled to receive signals from said trackat said selected location and to provide a second output voltageproportional to the impedance component across said tracks, said secondoutput voltage having an output vs. train distance curve of a slopesubstantially greater than zero over a train distance greater than saidfirst train distance, said first output voltage being indicative of thedistance of said train from said selected location over said greatertrain distance and being substantially free from the effect ofresistance variations in said tracks;

a differentiator circuit being coupled to said amplitude detector, andbeing responsive to said second output voltage of said amplitudedetector for providing a third output voltage indicative of theinstantaneous speed of said train at any position over said greatertrain distance, and

a summing amplifier being coupled to said amplitude detector and to saiddifferentiator circuit and being responsive to said third output voltageof said differentiator circuit and said first output voltage of saidquadrature detector,

said summing amplifier being adapted to provide a fourth output voltagewhich is a function of the time required for said train to arrive atsaid location from any train position over said greater train distance.

9. The system as defined in claim 8 and further comprising means coupledto said summing amplifier and to a warning device for utilizing saidfourth output voltage of said summing amplifier for operating a warningdevice at said location.

10. The system as defined in claim 8 and further comprising:

a reference voltage source adapted to provide an output voltage of apredetermined level; and

an amplitude comparator being coupled to said summing amplifier and saidreference voltage source and being responsive to said fourth outputvoltage of the said summing amplifier and to the output voltage of saidreference voltage source and being adapted to provide an output signalwhen the voltage provided by said summing amplifier exceeds the voltageprovided by said reference voltage source.

11. The system as defined in claim 8 wherein said source of AC signalsincludes:

an oscillator adapted to provide an output signal at a predeterminedfrequency; and

a power amplifier coupled between said oscillator and said tracks.

12. The system as defined in claim 11 and further including a band-passamplifier coupled between said track and said quadrature detector andsaid amplitude detector.

13. The system as defined in claim 12 and further comprising meanscoupled to said amplitude comparator for utilizing the output voltage ofsaid amplitude comparator for operating a warning device on saidlocation.

1. In a system for deriving from railroad tracks of low ballastinformation for predicting the time required for arrival at a givenlocation of a distant train which is moving on said track towards saidlocation, comprising: means for applying an AC signal of a constantcurrent level on said track from said location; means for deriving afirst voltage proportional to the reactance component across said trackssaid first voltage having an output vs. train distance curve approachingsubstantially zero slope in the vicinity of a first train distance fromsaid location; means for deriving a second voltage proportional to theimpedance component across said tracks having a time rate of changeindicative of the speed of said train approaching said location, saidsecond voltage having an output vs. train distance curve of a slopesubstantially greater than zero over a train distance greater than saidfirst train distance; said first voltage being indicative of thedistance of said train from said location over said greater traindistance and being substantially free from the effect of lumpedresistance in said track; means for differentiating said second voltageto obtain a voltage representative of the instantaneous speed of saidtrain at any train position over said greater train distance; and meansfor combining said first voltage and said differentiated voltage toprovide a third voltage which is a function of the time required forsaid train to arrive at said given location from any train position oversaid greater train distance.
 2. The system as defined in claim 1 andfurther comprising means for utilizing said third voltage for operatinga warning device at said location.
 3. The system as defined in claim 1wherein said means for deriving said first voltage is a quadraturedetector.
 4. The system as defined in claim 1 wherein said means forderiving said second voltage is an amplitude detector.
 5. The system asdefined in claim 1 wherein said means for differentiating is anoperational amplifier differentiator.
 6. The system as defined in claim1 and further comprising: means for providing a reference voltage; andcomparator means responsive to said reference voltage and said thirdvoltage provided by said combining means, said comparator means beingadapted to provide an output when said third voltage exceeds saidreference voltage.
 7. The system as defined in claim 6 and furthercomprising means for utilizing the output voltage of said comparatormeans for operating a warning device at said location.
 8. In a systemfor predicting the time of arrival of a train on a track comprising: asource of AC signals at a constant current level, said source beingcoupled across said tracks at a selected location; a quadrature detectorbeing coupled to receive signals from said track at said selectedlocation, said quadrature detector being adapted to proVide a firstoutput voltage proportional to the reactance component across said trackand having an output vs. train distance curve approaching substantiallyzero slope in the vicinity of a first train distance from said location;an amplitude detector being coupled to receive signals from said trackat said selected location and to provide a second output voltageproportional to the impedance component across said tracks, said secondoutput voltage having an output vs. train distance curve of a slopesubstantially greater than zero over a train distance greater than saidfirst train distance, said first output voltage being indicative of thedistance of said train from said selected location over said greatertrain distance and being substantially free from the effect ofresistance variations in said tracks; a differentiator circuit beingcoupled to said amplitude detector, and being responsive to said secondoutput voltage of said amplitude detector for providing a third outputvoltage indicative of the instantaneous speed of said train at anyposition over said greater train distance, and a summing amplifier beingcoupled to said amplitude detector and to said differentiator circuitand being responsive to said third output voltage of said differentiatorcircuit and said first output voltage of said quadrature detector, saidsumming amplifier being adapted to provide a fourth output voltage whichis a function of the time required for said train to arrive at saidlocation from any train position over said greater train distance. 9.The system as defined in claim 8 and further comprising means coupled tosaid summing amplifier and to a warning device for utilizing said fourthoutput voltage of said summing amplifier for operating a warning deviceat said location.
 10. The system as defined in claim 8 and furthercomprising: a reference voltage source adapted to provide an outputvoltage of a predetermined level; and an amplitude comparator beingcoupled to said summing amplifier and said reference voltage source andbeing responsive to said fourth output voltage of the said summingamplifier and to the output voltage of said reference voltage source andbeing adapted to provide an output signal when the voltage provided bysaid summing amplifier exceeds the voltage provided by said referencevoltage source.
 11. The system as defined in claim 8 wherein said sourceof AC signals includes: an oscillator adapted to provide an outputsignal at a predetermined frequency; and a power amplifier coupledbetween said oscillator and said tracks.
 12. The system as defined inclaim 11 and further including a band-pass amplifier coupled betweensaid track and said quadrature detector and said amplitude detector. 13.The system as defined in claim 12 and further comprising means coupledto said amplitude comparator for utilizing the output voltage of saidamplitude comparator for operating a warning device on said location.